What is atmospheric pressure?

Explain why:
$(a)$ The blood pressure in humans is greater at the feet than at the brain.
$(b)$ Atmospheric pressure at a height of about $6 \; km$ decreases to nearly half of its value at sea level,though the height of the atmosphere is more than $100 \; km$.
$(c)$ Hydrostatic pressure is a scalar quantity even though pressure is force divided by area.

Select the wrong statement about pressure.

$A$ spherical marble of radius $1 \, cm$ is stuck in a circular hole of radius slightly smaller than its own radius (for calculation purpose,both can be taken same) at the bottom of a bucket filled with water up to a height of $10 \, cm$. If the mass of the marble is $20 \, g$,then the net force on the marble due to water is close to:

$A$ $U$-tube containing a liquid moves with a horizontal acceleration $a$ along a direction joining the two vertical limbs. The separation between these limbs is $d$. The difference in their liquid levels is

Explain the pressure-measuring device at any point in a fluid in short.